Phase diagram analysis and thermophysical properties investigation of mixed molten salts with low liquid operating temperature for thermal energy storage

Molten salt is applied as an excellent medium in thermal energy storage (TES) systems. In this work, the LiNO3-KNO3 and LiNO3-KNO2 eutectic salts with the low liquid operating temperature were preferred using sub-regular solution model (SSM) and FactSage software. The liquid phase lines of the target systems were evaluated using thermodynamic methods and the phase diagram database of molten salts was supplemented. Combined with DSC experimental verification, it was found that the SSM method had high accuracy in calculating the eutectic point of the target molten salts. The melting points of LiNO3-KNO3 and LiNO3-KNO2 eutectic salts were 128.9 °C and 97.8 °C, respectively. The relevant tests indicated that the two eutectic salts had promising heat transfer and storage properties. The maximum sensible heat storage densities of LiNO3-KNO3 and LiNO3-KNO2 eutectic molten salts were as high as 592.2 kJ/kg and 667.3 kJ/kg in the liquid operating temperature range. Thermal stability of two molten salts under different thermal conditions was investigated. The test found that the changes in the NO2−/NO3− molar ratio in the LiNO3-KNO2 eutectic salt might affect its melting point under long-term high temperature conditions. This work not only provides efficient methods for screening novel molten salts, but also gives basic data for the industrial applications of molten salts for TES systems and energy conversion devices.